Rename functions/classes for elementwise operations

* Create CpuElementwise operator
* Rename kernel classes
* Make the kernels stateless

Partially implements: COMPMID-4003

Change-Id: I4ef9c61a3acc3ac5dbe46463d62dcb88a5face21
Signed-off-by: Sang-Hoon Park <sang-hoon.park@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4881
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Sheri Zhang <sheri.zhang@arm.com>
Reviewed-by: Georgios Pinitas <georgios.pinitas@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/docs/00_introduction.dox b/docs/00_introduction.dox
index cbdc983..f2dddbe 100644
--- a/docs/00_introduction.dox
+++ b/docs/00_introduction.dox
@@ -124,8 +124,8 @@
    - NEArithmeticSubtractionKernel
    - @ref NEPixelWiseMultiplication
    - @ref NEPixelWiseMultiplicationKernel
-   - @ref NEElementwiseDivision
-   - @ref NEDivisionOperationKernel
+   - NEElementwiseDivision
+   - NEDivisionOperationKernel
  - Interface change
    - Properly support softmax axis to have the same meaning as other major frameworks. That is, axis now defines the dimension
      on which Softmax/Logsoftmax is performed. E.g. for input of shape 4x5x6 and axis=1, softmax will be applied to 4x6=24 vectors of size 5.
@@ -569,7 +569,7 @@
      - @ref CLGEMMLowpMatrixMultiplyReshapedOnlyRHSKernel
      - @ref CLGEMMLowpMatrixMultiplyNativeKernel
      - @ref NEActivationLayer
-     - @ref NEComparisonOperationKernel
+     - NEComparisonOperationKernel
      - @ref NEConvolutionLayer
      - @ref NEDepthwiseConvolutionLayer
      - NEDepthwiseConvolutionLayer3x3Kernel
@@ -821,7 +821,7 @@
  - New Neon kernels / functions:
     - @ref NETileKernel / @ref NETile
     - @ref NEFuseBatchNormalizationKernel / @ref NEFuseBatchNormalization
-    - @ref NEElementwiseOperationKernel
+    - NEElementwiseOperationKernel
     - @ref NEElementwiseMax
     - @ref NEElementwiseMin
     - @ref NEElementwiseSquaredDiff
@@ -842,7 +842,7 @@
     - @ref NEGatherKernel / @ref NEGather
     - @ref NEElementwiseComparison
     - @ref NEElementwiseComparisonStatic
-    - @ref NEComparisonOperationKernel
+    - NEComparisonOperationKernel
     - @ref NEElementwiseDivision
  - New OpenCL kernels / functions:
     - @ref CLSelectKernel / @ref CLSelect